Electrochemical capacitor with electrode material for energy storage

ABSTRACT

Iron oxide film directly grown on iron, steel, or other substrates by chemical or electrochemical oxidation is a promising material for energy storage through surface adsorption of static charges. As the electrode materials of energy-storage devices, the iron oxide has a chemical composition of Fe x O y H z , where 1.0≦x≦3.0, 0.0≦y≦4.0, and 0 0≦z≦1.0. An aqueous or organic solution of a metallic salt including sulfates, sulfites, hydroxides, chlorides, phosphates and nitrates is used as electrolyte for the electrochemical devices. Cyclic voltammetry indicates that the iron-oxide electrodes in the electrolytes can store charges as high as 0.5 F/cm 2  or 320 F/g of the electrode materials. Electrochemical capacitors using the iron oxide as the electrode material is an economical and viable power source for use in portable electronics, power tools, and electrical vehicles.

FIELD OF THE INVENTION

[0001] The present invention relates to an energy storage device thatcan enhance the performances of batteries in numerous applications. Morespecifically, the present invention relates to an iron oxide compoundused as the electrode material for supercapacitors.

BACKGROUND OF THE INVENTION

[0002] Supercapacitor is also known as ultracapacitor or electric doublelayer capacitor. In rigid terms, though there is some distinction amongthem, they all can store a large quantity of charges up to severalthousands farad (F) in compact sizes. Furthermore, they all have highpower density (>1 KW/Kg), high charge-discharge life (>10⁴ cycles), andhigh discharge efficiency (>90%). The high power density ofsupercapacitor derives from its quick-discharge characteristics inconjunction with large capacity of energy-storage. Such high powerdensity imparts supercapacitors and the like a unique role as thepeak-current provider in hand-held electronic devices, portable powertools electrical vehicles (EVs) and automatic actuators.

[0003] All primary and secondary batteries are generally used to deliversmall currents for lengthy times. This is due to the energy storage ofbatteries involves bulk oxidation-reduction which is thermodynamicallycontrolled. Some batteries, such as lead-acid batteries, are capable ofdischarging quickly, delivering an instant large current greater than100A in applications like the ignition of automobiles. Nevertheless, thebatteries can only provide such large output at very short periods andinfrequent repetitions, otherwise the batteries will soon be drained ordamaged. In addition to miniaturization of the consumer electronics withinevitable shrinkage of batteries, the EVs are in urgent need forreducing oil consumption and air pollution, batteries should work inparallel with supercapacitors to fulfill the power requirements thatbatteries alone could not offer. In the parallel connection of batteriesand supercapacitors, the latter can virtually provide any peak-currentrequired repeatedly. This allows the batteries to discharge at ratedcurrents, and, as a consequence, the use-time and the life-time ofbatteries are prolonged. The aforementioned effect is called loadleveling. In light of no limitation, except voltage (not to exceed therated value), on the charging mode of device, supercapacitors are a moreversatile energy-storage device than battery. Especially in theregenerative braking of EVs, supercapacitors can quickly and safely savethe residual kinetic energy of EVs for later use.

[0004] The utilization of supercapacitor in the energy-management systemof batteries has been validated. However, the present market prices ofsupercapacitors, as well as their dimensions and specifications, preventthem from general acceptance. Regardless of their merits,supercapacitors must offer an affordable price to be commerciallyviable. To lower the cost of supercapacitors, an inexpensive and readilymade electrode material should be found. The most frequently usedelectrode materials for supercapacitors include activated carbons andmetal oxides. Metal oxides are superior to activated carbons in energydensity, conductivity and workability. Oxides of various transitionmetals including ruthenium, rhodium, iridium, titanium, cobalt,molybdenum, tungsten, vanadium, manganese and nickel are investigated.Ruthenium oxide (RuO₂), either in crystalline or amorphous state, andiridium oxide are determined to have a specific capacitance in the rangeof 100-750 F/g, which is equivalent to or three-time-higher than thevalue attainable from carbons. Ruthenium is a by-product in theextraction of platinum, hence Ru is rare and expensive. Cost-wise, RuO₂is unsuitable as the electrode material for making supercapacitors forgeneral use. Other compounds such as sulfides, hydrides and nitrides ofthe aforementioned metals, iron and lead sulfides, as well as molybdenumand tungsten carbides and borides have been tested as the electrodematerial for electrochemical capacitor. Whereas the energy-storagecapability of the above materials is generally low, the cost of thestarting metals or precursors for producing the minerals is considerablyhigh, and the fabrication procedures of the metal oxides are costly aswell. Clearly, it requires a more economical and easy-of-preparationelectrode material than the above substances to solve the cost problemof supercapacitors for wide applications.

SUMMARY OF THE INVENTION

[0005] As discussed in greater detail below, the present inventionprovide the most economical material of the existing electrode materialsfor energy storage through surface adsorption of static charges. Theprimary object of the present invention is to provide supercapacitorscomprising iron oxide as the active material of electrodes of thesupercapacitors. Iron oxide with a chemical composition ofFe_(x)O_(y)H_(z), where 1.0≦x≦3.0, 0.0≦y≦4.0, and 0.0≦z≦1.0, can beyielded in a thin film on iron, steel, or other substrates. Inconjunction with suitable electrolytes, the electrode materials showcapacitance of as high as 0.5 F/cm² or 320 F/g.

[0006] Another object of the present invention is to demonstrate thatthe black iron (II,III) oxide or magnetite (Fe₃O₄) is the majorcomponent of Fe_(x)O_(y)H_(z) to be responsible for the highenergy-storage capacity of iron oxide. Other form of iron oxide such asFeO, Fe₂O₃ or FeO(OH) is likely present with the magnetite.Nevertheless, its presence appears to cause no adverse effects.

[0007] Yet another object of the present invention is to provide adirect growth of iron oxide film on iron, steel or other substrates.Methods of one-step preparation include chemical oxidation,electrochemical oxidation, dip-coating, and electrophoretic deposition.Among them, chemical oxidation appears to be the most convenient way. Assoon as the iron-oxide film is attained, the film-coated substrates areready to form supercapacitors. Neither binder nor additionalelectrode-fabricating equipment is required in the present invention.Supercapacitors of the present invention can be prepared in simpleprocedures and no binder is needed, the present invention can furtherreduce the preparation cost of supercapacitors.

[0008] Still another object of the present invention is to provide ironoxide as the sole or partial ingredient of the electrode materials forsupercapacitors. Iron oxide may be used alone, or it may mix withcarbons, metal powders or mineral particles to form a compositeelectrode for supercapacitors. Iron-oxide film may also be formed on aporous support such as Sb-doped SnO₂. The aforementioned combinationsutilize the low-cost iron oxide to prepare affordable supercapacitors.

[0009] The last object of the present invention is to provide anenvironment-friendly material, iron oxide, for fabricatingsupercapacitors. Iron oxides are commonly present in numerous ores onearth. Scraps from the spent iron-oxide-electrodes of supercapacitorswill cause no harm to the environments. Furthermore, theiron-oxide-electrodes are easy to regenerate and the substrates may beused repeatedly.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The present invention is better understood when read inconjunction with the following drawings, in which:

[0011]FIG. 1 is the X-ray diffraction pattern of a Fe₃O₄ film electrodeprepared by chemically oxidizing a carbon steel substrate in a boilingsolution containing 1000 g NaOH, 12 g NaNO₃ and 12 g Na₂Cr₂O₇ in 1 literde-ionized water. The arrowed reflection are due to Fe₃O₄, while thosemarked with ▪ are due to the substrate, Fe.

[0012]FIG. 2 is a cyclic voltammogram of two electrodes of 2 cm×2 cmFe₃O₄/Fe substrate under 50 mV/sec scanning rate in 0.1M Na₂SO₄ and 0.5MKOH.

[0013]FIG. 3 is a cyclic voltammogram of two electrodes of 2 cm×2 cmFe₂O₃/Fe substrate under 50 mV/sec scanning rate in 0(0.1M Na₂SO₄ and0.5M KOH.

[0014]FIG. 4 is a cyclic voltammogram of Fe₃O₄ film electrodes in 1MNa₂SO₄ aqueous solution under 20 mV/sec scanning rate.

[0015]FIG. 5 is a cyclic voltammogram of electrodes consisting of amixture layer of iron oxyhydroxide and oxide deposited on titaniumsubstrates in 1M Na₂SO₄ aqueous solution under 20 mV/sec scanning rate.

[0016]FIG. 6 is a constant-current charge-discharge plot of electrodesconsisting of a mixture layer of iron oxyhydroxide and oxide depositedon titanium substrates in 1M Na₂SO₄ aqueous solution under a currentdensity of 5 mA/cm₂.

[0017]FIG. 7 is a self-discharge curve of a primitive supercapacitorcontaining two serially connected cells. Each cell consists of two pairsof 8 cm×8 cm Fe₃O₄/Fe electrodes connected in-parallel.

[0018]FIG. 8 is a group of discharge curves of a primitivesupercapacitor using 8 cm×8 cm Fe₃O₄/Fe electrodes under variousconstant currents. The prototype device is rated as 2.5V×0.1F, and LEDis the abbreviation of the assignee.

DETAILED DESCRIPTION OF THE INVENTION

[0019] Depending on the electrode materials, supercapacitors may utilizetwo different mechanisms, double layer (DL) or surfacereduction-oxidation, to store electric charges and form double layercapacitance or pseudocapacitance. A DL of opposite charges isautomatically formed on the solid-liquid interface when a conductor isplaced in an electrolyte solution, which blocks the diffusion of ions orspecies to the conductor for analysis. DL is thus minimized from theinterface of solid and liquid in electrochemical analyses. However, theDL structure is deliberately maximized to store static charges to formDL capacitance in supercapacitors. There is no charge transfer in DLcapacitance, yet the psudocapacitance comes from faradaic reactionsinvolving surface or adsorbed species at the electrode-electrolyteinterface. It involves faradaic charge transfer occurring at theelectrode surface rather than within the bulk as in galvanic cells.Pseudocapacitance can be 10 to 100 times greater than DL capacitance.Both DL capacitance and pseudocapacitance are related to physi-sorptionwhere charges are quickly stored and released, which are kinetic balanceand it is the reason why the supercapacitors have high power density.

[0020] The energy stored in capacitors can be determined by thefollowing formula:

E=½ CV ²  (1)

[0021] where E is energy in joule, C is capacitance in farad (F), and Vis the working voltage of capacitor in volt. Because of the second-powerof V, capacitors are normally designed to work at as high voltage aspossible so that they can store more energy. Even without meticulouspackaging, the bare electrodes of supercapacitors can be evaluated usingcyclic voltammetry (CV). From CV graphs, useful information regardingcapacitance, kinetics, stability and cycle-life of the electrodesstudied can be attained. For instance, the capacitance per electrodematerial C_(el) can be assessed by the following equation:

C _(el)=2[(i _(c) +i _(a))/2]/(dV/dt)  (2)

[0022] where i_(c) and i_(a) and are cathodic (reduction) and anodic(oxidation) current at 0.0 V, respectively, in Ampere, and dV/dt is thevoltage scanning rate in volt/sec. Considering a cell is formed by twoserially connected electrode-capacitors, a multiplication of 2 appearsin equation (2).

[0023] As mentioned before, thin films of iron-oxide can be mostconveniently prepared by chemical oxidation. For example, magnetite(Fe₃O₄) is formed rapidly in a strong alkaline solutions in the presenceof an oxidizing agent such as NaNO₃ at temperatures above 100° C. In thereactions, Na₂FeO₂ and Na₂Fe₂O₄ are formed first, then they react witheach other to form thin Fe₃O₄ film on iron substrate as described below:

3Fe+NaNO₂+5NaOH═3Na₂FeO₂+H₂O+NH₃  (3)

6Na₂FeO₂+NaNO₂+5H₂O═3Na₂Fe₂O₄+7NaOH+NH₃  (4)

Na₂FeO₂+Na₂Fe₂O₄+2H₂O═Fe₃O₄+4NaOH  (5)

[0024] In the above reactions, iron is initially dissolved in thealkaline solution to form saturated solution of iron oxide at thesolid-liquid interface. Therefrom, crystalline seeds of iron oxide areformed on some area of the iron substrates, and further growth of theseeds results in a continuous film of magnetite. As magnetite film growsfrom nm thickness to above 2 μm, it will change from lustrous pale-blueto dull-black color. Further oxidation of magnetite will convert theblack oxide into reddish-brown iron oxide or hematite (Fe₂O₃). Ironoxide formed in aqueous solutions is likely in hydrous states and isbest described by a chemical composition of Fe_(x)O_(y)H_(z), where1.0≦x≦3.0, 0.0≦y≦4.0, and 0.0≦z≦1.0.

[0025] We have prepared thin black film of iron-oxide on carbon steelusing chemical oxidation. When the film is subjected to X-raydiffraction (XRD) analysis, the result is shown in FIG. 1. It indicatesthat the film is predominantly magnetite (Fe₃O₄). Plates of 8 cm×8 cmFe₃O₄/Fe electrodes are also used to prepare primitive supercapacitorsfor assessing their commercial potential. The following examples onlyillustrate the present invention provides a method for preparing thiniron oxide films and shows use of the thin iron oxide films forsupercapacitors has promising commercial quality.

[0026] The present invention will be better understood from thefollowing example which are merely for the purposes of illustration andby no means of any limitation therefore.

EXAMPLE 1

[0027] 0.1 mm-thick carbon steel plates were cut to 2 cm×2 cm dimension,and the samples were cleaned with acid, rinsed with de-ionized water,and finally polished with sand-paper. Then the cut plates were placed ina boiling (ca 140° C.) aqueous solution containing 1 g NaOH/ml H₂O and12 g KMnO₄/I H₂O. After 4 min of cooking, a uniform black film wasformed on the substrates. Following the removal of plates from thesolution, the substrates were rinsed and dried to yield the Fe₃O₄electrodes. Electrodes as prepared are ready for analyses or for makingsupercapacitors. With addition of de-ionized water and the oxidizingagent, the same oxidizing bath can be used repeatedly. Cyclicvoltammogram (CV) was used to characterize the electrodes, and a resultof the analyses is shown in FIG. 2.

[0028]FIG. 2 shows the 10th CV graph of two free-standing electrodes ofblack Fe₃O₄ thin-film on 2 cm×2 cm iron substrate in an aqueous solutioncontaining 0.1M Na₂SO₄ and 0.5M KOH under 50 mV/sec scanning rate. Noreference electrode is used in the analysis. Except slight tilt at bothends, the CV loop is a nice rectangle, which is a typical capacitorbehavior, with rapid change of current at voltage reversal. Furthermore,the voltammogram remains the same shape in many cycles of voltagescanning. This indicates that the iron oxide has good reversibility,conductivity and sorption-desorption characteristics. From FIG. 2 andequation (2), the iron oxide electrodes produced are estimated to havecapacitance of 0.03 F/cm².

EXAMPLE 2

[0029] Thin Fe₂O₃/Fe electrodes were prepared according to example 1except the cooking time is extended to 20 minutes. At that time,reddish-brown color on the substrates was also observed. Fe₂O₃/Feelectrodes were also analyzed by CV as example 1, and one of the resultis shown in FIG. 3. It is clearly seen that the CV loop of Fe₂O₃ is nota normal behavior of capacitors. Thus, the capacitance of the Fe₂O₃electrode material can not be estimated precisely. However, there is asignificant difference between the capacitance of the two iron oxides,and it may be interpreted from their difference in crystal structure andconductivity.

[0030] Fe₃O₄ is an inverse spinel consisting of two oxidation states ofiron, Fe(II) and Fe(III), with the less abundant Fe(II) restricted tofour-fold tetrahedral sites and the more abundant Fe(III) distributedevenly between the tetrahedral sites and six-fold octahedral sites,which gives Fe₃O₄the formula as

[Fe³⁺]^(tet.)[Fe²⁺Fe³⁺]^(oct.)O₄

[0031] There is electron hopping between Fe(II) and Fe(III), whichimparts Fe₃O₄ as a semiconductor with resistivity of 10⁻² Ω-cm. On theother hand, Fe₂O₃ is a rhomohedral oxide consisting of a hexagonal closepacked oxygen array with two thirds of the octahedral intersticesoccupied by Fe(III). There is no movement of electron in the structureand Fe₂O₃ is an insulator with a band gap of 3.1 eV. In light of Fe₂O₃being used as an solid adsorbent for removing hazardous gases such asnitrogen oxides and sulfur dioxide, surface area of the electrodesshould not be responsible for the great capacitance difference betweenFe₃O₄ and Fe₂O₃. Instead, the difference in the conductivity of the ironoxides should be the cause.

EXAMPLE 3

[0032] 1 cm×1 cm carbon steel substrates were cooked in one literde-ionized water containing 1000 g NaOH, 12 g NaNO₃ and 12 g Na₂Cr₂O₇ at135-145° C. for 20 minutes to produce a 3 μm composite iron-oxide layerpredominantly in Fe₃O₄. A sandwich-type cell was prepared by disposing aglass-fiber separator soaked with 1M Na₂SO₄ electrolyte between twoiron-oxide electrodes. The cell was analyzed by CV using 20 mV/secscanning rate between −0.8 volt and +0.8 volt. FIG. 4 shows the resultof CV graph. By equation (2), C_(el) of the electrode material studiedis determined to be 0.38 F/cm². Assuming the density of the porous oxidelayer is 4.0 g/cm³, and using the known layer thickness of 3 μm, theabove capacitance is converted to a specific capacitance of 320 F/g ofthe electrode material.

EXAMPLE 4

[0033] An electrochemical capacitor was built according to example 3except that 0.5M Na₃PO₄ aqueous solution was used as the electrolyte.The capacitance of the electrode material was determined using the sameprocedures as example 3 and was found to be 0.08 F/cm².

EXAMPLE 5

[0034] An electrochemical capacitor was assembled according to example 3except that iron plates were used as the substrates. As demonstrated inexample 3, the cell was examined using CV and the same electrolyteexcept higher scanning rate of 50 mV/sec was used. The capacitance ofthe electrode material was determined to be 0.02 F/cm².

EXAMPLE 6

[0035] An electrochemical capacitor was made according to example 3except that an aqueous solution containing 1M Na₂SO₄ and 0.001M KOH wasused as the electrolyte. CV measurement was conducted using the sameprocedures as example 3, and C_(el) was found to be 0.2 F/cm².

EXAMPLE 7

[0036] 1 cm×1 cm iron substrates were oxidized in one liter watercontaining 1000 g NaOH, 12 g NaNO₃ under a constant anodic current of 9mA for 3 minutes. A sandwich-type cell was prepared by disposing aglass-fiber separator soaked with 0.1M Na₂SO₄ electrolyte between twoanodized electrodes. The cell was analyzed by CV using 20 mV/secscanning rate between −0.8 volt and +0.8 volt, and C_(el) was determinedto be 0.05 F/cm².

EXAMPLE 8

[0037] 1 cm×1 cm titanium substrates were first coated with anconductive porous Sb-doped SnO₂ layer. The latter was then electroplatedin 1M aqueous FeSO₄ solution under a constant current of 195 mA for 5minutes so that iron was deposited within the interstices, as well as onthe surface of the porous layer. There are two functions for the SnO₂layer: to provide porous sites for the formation of iron and iron oxideparticles, and to provide a highly conductive pathway for the ironoxide. The electroplated substrates were then thermally oxidized in 0.01torr air at 700° C. for 3 minutes. An electrochemical capacitor wasassembled according to example 3, CV measurement was conducted as well.C_(el) of the electrode material was found to be 0.04 F/cm².

EXAMPLE 9

[0038] Two SnO₂-coated titanium substrates prepared according to example8 were immersed in 1M FeSO₄ aqueous solution at pH 8. A stream of oxygengas was bubbled through the solution for 30 minutes to yield a compositelayer of yellowish iron oxyhydroxide [FeO(OH)] and black iron oxidewithin the interstices, as well as on the surface of the porous SnO₂layer. Using the iron-oxide electrodes as prepared, an electrochemicalcapacitor was assembled according to example 3, CV measurement wasconducted and shown in FIG. 5. It is a quasi-rectangle CV loopindicating that the electrode materials have good kinetic reversibilityand conductivity. C_(el) of the electrode materials was found to be 0.04F/cm². When the same cell was subjected to constant-current-density of 5mA/cm² charging and discharging, it yielded linear potential-versus-timecurves as shown in FIG. 6. Swift charge-discharge as seen is a typicalcharacter of capacitors. Both FIGS. 5 & 6 demonstrate that the presentinvention is full of commercial merits.

EXAMPLE 10

[0039] 8 cm×8 cm Fe₃O₄/Fe electrodes were prepared according to example1, and a unit cell was constructed simply by placing four pieces ofelectrodes in a regular plastic bag with a Manila paper disposed betweenevery two electrodes. Without using spot or laser welding, theelectrodes were clamped in parallel connection. After an aliquot of anaqueous solution containing 0.1M Na₂SO₄ and 0.5M KOH was put into thebag, it was sealed using a heat sealer. Neither additionalencapsulation, nor compression was applied to the electrodes forintimate contact. Two loose unit-cells were connected in series to formprimitive supercapacitors. Then, alligator clips were connected to theanode and cathode of supercapacitor for electrochemical and electricalcharacterizations. FIG. 7 shows a self-discharge curve of the primitivedevice after being charged to 2.8 volt. Initially the voltage of thedevice decreases very rapidly then levels off. Such behavior is commonlyobserved for regular capacitors as well as supercapacitors includingcommercial products. Nevertheless, the present invention shows a highself-discharge rate that is in accordance with many loose ends in thecurrent cell-design. FIG. 8 contains the discharge curves of the fullycharged device under various constant currents. Therefrom, thespecifications of the prototype are extracted and listed in thefollowing table: Maximum Working Capacitance ESR Weight DimensionsVoltage (V) (F) (mΩ) (g) (mm) 2.5 0.1 84.5 57.3 121 × 109 × 1.7

[0040] Despite the primitive construction of the device as describedabove, the present invention has demonstrated promising qualities forcommercial use. Particularly, the prototype shows low ESR (equivalentseries resistance) which is very important in high frequency and highpower applications, and thin cell-thickness (1.7 mm) which is incompliance with the miniaturization of electronic devices. FIG. 8 alsoshows that the prototype is capable of delivering a peak current as highas 10A, and that is useful in applications requiring pulse powers.

[0041] Although preferred embodiments have been described to illustratethe present invention, it is apparent that changes and modifications inthe described embodiments cab be carried out without departing from thescope of the invention intended to be limited only by the appendedclaims.

1. An electrochemical capacitor, comprising an electrode containing as least one material being hydrated iron compound having a chemical composition of Fe_(x)O_(y)H_(z), where 1.0≦x≦3.0, 0.0≦y≦4.0, and 0.0≦z≦1.0, said electrochemical capacitor is known as supercapacitor, ultracapacitor, or electric double layer capacitor.
 2. The electrochemical capacitor of claim 1, wherein said Fe_(x)O_(y)H_(z) is Fe₃O₄.
 3. The electrochemical capacitor of claim 1, wherein said Fe_(x)O_(y)H_(z) is conductive and magnetic.
 4. The electrochemical capacitor of claim 3, wherein said Fe_(x)O_(y)H_(z) has a conductivity of no less than 10⁻² Siemen/cm.
 5. The electrochemical capacitor of claim 3, wherein said Fe_(x)O_(y)H_(z) has a magnetic flux density of no less than 10 Gauss.
 6. The electrochemical capacitor of claim 1, wherein said Fe_(x)O_(y)H_(z) is directly grown on a conductive substrate which is first plated with a metallic iron.
 7. The electrochemical capacitor of claim 1, wherein said Fe_(x)O_(y)H_(z) is coated on a conductive substrate. 